In a conventional power-assisted vehicle braking system for passenger cars and light trucks, an operator depresses a brake pedal to progressively actuate a master hydraulic brake cylinder that is mechanically coupled to the brake pedal. The applied pedal force is amplified or “power assisted” by either a vacuum booster or a hydraulic booster coupled to the master cylinder, whereupon the master brake cylinder builds up a correlative amount of pressure in the brake lines to actuate the wheel brakes of the vehicle. More specifically, the pressurized fluid causes brake pads to press against a rotor or brake shoes against a drum coupled with the wheel, thereby retarding rotation of the wheel. In this manner, such conventional power-assisted vehicle braking systems achieve a wheel brake damping force sufficient to decelerate the vehicle.
Many vehicles are also equipped with electronically-assisted “active braking” systems, or electronic braking systems “EBS”, in which the supply of pressurized fluid to each individual wheel brake is also moderated under a microprocessor control to prevent wheel lockup (in so-called anti-locking braking systems or ABS) and undesirable wheel slip (in so-called automatic slip control or ASR). Other variations and embodiments of such systems are: traction control systems (TCS), electronic stability programs (ESP), and active rollover protection (ARP). These active braking systems typically include wheel speed sensors to identify the wheel lockup and wheel slip. Additionally, solenoid-operated regulating valves selectively control the supply to and release of pressurized fluid to the brake cylinder of each wheel. Furthermore, these systems typically include a motor-driven pump, disposed on the “back” or supply side of the power-assisted master cylinder, which is capable of providing additional hydraulic fluid pressure and volume to the wheel cylinders during ABS operation.
In another known system, the brake pedal is connected to and triggers the activation of an active booster. Under a first set of operating conditions, such as when relatively small forces are applied to the brake pedal, the force applied to the brake pedal is generally proportional to the force applied to the master cylinder. However, under a second set of operating conditions, such as when relatively large forces are applied to the brake pedal, the active booster causes the master cylinder to be “boosted” by applying a force onto the master cylinder that is greater than the force applied to the pedal. The active booster includes at least first and second chambers that have lower pressures than the ambient during the first set of operating conditions. The first and second chambers are separated by a movable diaphragm that is connected to the master cylinder. Once the brake pedal has traveled a predetermined distance such that the second set of operating conditions occur, the first chamber is filled with ambient air, thereby applying a force to the diaphragm and to the master cylinder that is greater than the force applied to the brake pedal.
However, the system described above may not be fully operational during a mechanical failure, such as a vacuum failure, causing the system to have a low response time to system changes or failures. Additionally, the above-described system may not be quickly responsive to other system changes, such as a change in operating conditions. Furthermore, the above-described system may not meet Federal requirements for resulting deceleration in response to a given applied muscle force, or the system may require undesirably expensive and/or bulky components to comply therewith.
Therefore, it is desirous to provide a system that includes secondary, backup system controls that are quickly and fully responsive to system changes and/or system failures.